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Authors
Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 45, No 3 (1995), Pagination: 301-310
Abstract
Critical studies of a part of Singhbhum basin around Bisrampur, Singhbhurn district, Bihar, show that the metasediments (considered to belong to the Dhanjori Group) lie unconformably above the Singhbhurn Granite. However a few tongues of the granite that occur within the metasediments have been explained as thrust slices of the basement within the cover rocks. Though three phases of deformation have been recorded here, and the general outcropgattern has been largely controlled by the second phase of deformation, there has not been any marked change in the spatial disposition of the coarser clastics. The shape of pebbles within the conglomerate, though often triaxial, cannot be correlated with the deformational history. This calts for original triaxiality of the pebbles. The high proportion of clastmatrix indicates that they were deposited under a two phase flow mechanism: a gravity mass flow aided by a high turbulent flow. Relatively high amounts of solid material in the fluid with variable particle size can create such a turbulent motion and the resultant scattered dispersion possibty accounts for the randomness in pebbleorientation. Accumulation of huge coarse clastics is, however, attributed to gravity mass flow. The general lack of turbidite structures in these litho-units can be attributed to flow inhomogeneity, flow velocity, nature of depositional substrate, relative significance of head-versus-body spilling from turbidity current. Presence of primary structures in arenaceous rocks and occurrence of these rocks as isolated bodies point to vigorous reworking and redeposition of the bottom sediments after cessation of the main flow. The huge pile of the rnicaceous phyllites probably represents relatively rapid deposition of silt grains from a highly concentrated mud-dominated turbidity current or very fluid silty debris flow deposited during the waning phase of turbidity flow. The entire load was deposited possibly over a shallow shelf edge. Retention of the original grain fabrics and the sedimentary structures in these metasediments, even though they are very close to a regionally developed shear zone, points to the existence of focally developed, sheltered low stress regimes provided by the Granite basement.
Keywords
Sedimentary Structures, Sedimentation, Dhanjori Group, Singhbhum, Bihar.